DESCRIPTION This program, referred to as Middle School Physical Activity and Nutrition (M-SPAN), is designed to change the school environment in such a way as to promote increased physical activity and reduced dietary fat consumption by students. The specific interventions to be carried out in the school are derived from theoretical frameworks representing social ecology and applied behavior analysis. During the two-year intervention PE teachers will modify classes in an effort to induce more activity and increase other opportunities on campus for physical activity. In addition, food service workers will be trained to recommend lower fat foods, and food vendors will be encouraged to supply low-fat food choices in cafeterias, stores, and vending machines. Other activities will be devised to work with students who do not buy lunches at school. Student Health Committees will be formed to encourage appropriate exercise and dietary habits among their classmates. Outcome measures will involve direct measures of student behavior related to physical activity and diet. Observations will be obtained for baseline, intervention, and maintenance periods. A total of 24 schools will be divided into experimental and control conditions and will be the units of randomization and statistical analysis.